Terpenoid Synthesis from Isoprene

Terpenoid Synthesis from Isoprene.—Syntheses of the useful synthons (33) and (34) have been reported. The vinyl-oxygen bond of diketen is selectively cleaved by Me3SiCH2MgCl-NiCl2 to yield the synthon (35) which is readily converted into the dicopper dienolate (Vol. 7, p. 14) and prenylated to yield (36 X = C02H) after hydrolytic work-up and desilylation (36 X = C02H) yields 

Much of the telomerization work this year is either closely related or similar to work reported previously. This includes tail-to-tail isoprene dimerization [Pd(PEt)j-C02] to (37) which is extensively isomerized on prolonged reaction,  

Kitamoto, K. Kameo, S. Terashima, and S.-I. Yamada, Chem. and Pharm. Bull. (Japan), 1977,25, 1273. 

Fujino, M. Wakimasu, M. Mano, K. Tanaka, N. Nakajima, and H. Aoki, Chem. and Pharm. Bull. (yopan), 1976, 24, 2112. 

Further syntheses with isoprene units are discussed in the next section. 

Terpenoid Synthesis from Isoprene.—Interest continues in new syntheses of iso-prene and its derivatives the dioxan (37) is obtained108 in good yield by the Prins reaction of methylallyl chloride with formaldehyde (cf. Vol. 5, p. 8) free-radical addition of isopropyl alcohol to vinyl acetate yields compound (38) which gives isoprene by acid-catalysed reaction over alumina.109 (Z)-2-Methylbut-2-en-l-ol and dimethylallyl alcohol are readily available from frans-crotyl alcohol.110 

Isoprene reacts with dimethylallyl chloride in the presence of CuC1-A1203 to yield geranyl chloride, neryl chloride, lavandulyl chloride, and myrcene(12) 111 reaction of (39 X = C1) with antimony trichloride gives (E)- and (Z)-2,6-dimethylocta-2,5,7-trien-4-one.111 (E)- and (Z)-3,7-Dimethylocta-3,6-dien-l-ols, geraniol (8), nerol (18), and the dienol (40) are prepared from isopentenyl acetate and prenyl chloride 

Omichi, T. Miyakoshi, and S. Saito, Nippon Kagaku Kaishi, 1975, 714. 

Fukunishi, 1. Naito, and F. Mashio, Nippon Kagaku Kaishi, 1974, 2014. 

Terpenoid Synthesis from Isoprene.—Co-oxidation of thiophenol and isoprene with oxygen yields the synthons (9) and (10) in useful yields and the isoprene epoxide (11) is a useful hemiterpenoid synthon with carbanions.  

The full paper on Cookson s syntheses of ocimenones, filifolene, and the tagetones has appeared (Vol. 5, p. 8). ° One-pot syntheses of the predominantly trans-isomers ( 80%) of (12 X = CH2OH) and (13 named lavandurol), (12 X = CO2H), and (12 X = CHO) result from [(77 -C5Hs)2TiCl2]-catalysed regioselective isoprene 

Sugiyama, A. Kobayashi, and K. Yamashita, Agric. and Biol. Chem. (Japan), 1975, 39, 1483. 

Takabe, K. Hashimoto, T. Katagiri, and J. Tanaka, Asahi Garusu Kogyo Gijutsu Shoreikai Kenkyu Hokoku, 1974, 25, 163 (Chem. Abs., 1976, 84, 44 376). 

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As part of a general investigation of terpenoid synthesis from isoprene units it has been shown that condensation of limonene (46) with senecioyl chloride in the presence of stannic chloride, followed by dehydrochlorination, provides a new simple synthetic route to a mixture of cis- (47) and frans-atlantones (48).23a Using acetyl chloride instead of senecioyl chloride in the synthetic sequence provides a C12 trisnor-sesquiterpenoid (49) which co-occurs with trans-atlantone in the essential oil of Cedrus atlantica.23b... 

Terpenoid Synthesis from Isoprene.—Some prenylation procedures have been reported earlier. An efficient conversion of prenyl chloride into prenyl acetate has been described and the synthon (31 X = OH) (Vol. 7, p. 10) has... 

Telomerization of Isoprene.—Reviews have appeared on isoprene and chloro-prene, and on the complex reactions of isoprene to form terpenoids (in Japanese). Isoprene reacts with magnesium, especially in the presence of Lewis acids, and the resulting complex gives adducts with aldehydes. As usual in this type of reaction, a very complex mixture is obtained. The palladium-chloride-catalysed reaction of isoprene with acetic acid gives different products in different solvents. Monomers predominate in benzene [2-methylbut-2-enyl acetate (5) and 3-methylbut-2-enyl acetate (6)] while dimers [(7), (8), neryl (9), and geranyl (10) acetates] tend to be formed in tetrahydrofuran. Further details of the synthesis of Cio alcohols from isoprene and naphthyl-lithium are available, as well as of the in situ oxidation,but there is little of novelty (see Vol. 1, p. 17). 

Excellent reviews have appeared on the synthesis of monoterpenoids,1 of cannabinoids,2 and of the use of isoprene in terpenoid synthesis,3 and also on terpenoids from marine sponges,4 the base-catalysed isomerization of mono-... 

The terpenoid portion of TIAs is derived from secologanin, whose monoterpene precursor geraniol, is produced by the recently discovered Rommer or triose phosphate / pyruvate pathway, responsible for the synthesis of isoprenes like geraniol in the plastids [58-60]. 

One of the monomers used to make synthetic rubber was also introduced in Karlsruhe and started a whole series of 53 publications [6] Uber die Darstellung von Isopren aus Terpenkohlenstoffen (On the synthesis of isoprenes from terpenoide hydrocarbons) [10]. In this work, 1,4-polyisoprene is simply an eight-member ring built of two monomers. 

The selective telomerization exclusively to natural type terpenoids with a head-to-tail connection has not been achieved so far, but some interesting initial steps have been done. In 1975 Hidai et al. had investigated the synthesis of the terpenoid citronellol starting from isoprene [30]. They reacted isoprene and methanol, yielding the telomer 1-methoxy-... 

Iridoids are a large and stmcturally diverse class of secondary metabolites of monoterpenoid origin [10, 11]. Their sttuc-tural parent system is the iridane skeleton, which is derived from geraniol (6) by a cyclization pathway that is mechanistically different to the cyclization reactions that are usually found in classical terpene chemistry [1, 12], Further enzymatic transformations then give the fundamental iridoid skeleton or the thereof derived secoiridoid motive (with an alternative connectivity as shown in Scheme 6.10), which often serves as a building block for the synthesis of more complex monoterpenoid indole alkaloids [1]. Noteworthy, hereby the isoprene mle is often not fulfilled anymore as the terpenoid parts of such compounds sometimes contain nine carbon atoms only due to a decarboxylation step somewhere in the sequence. Very often iridoids and secoiridoids are present as glycosides in nature. 

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